This invention relates to a time base connector and, more particularly, to such a time base corrector which is readily adapted to correct or compensate time base variations in a reproduced digital signal wherein the possibility of errors is substantially minimized.
It is known that, when a signal is recorded on, for example, magnetic tape and subsequently reproduced therefrom, the timing, or clocking of the reproduced signal may differ from that of the originally recorded signal. For example, changes in the motion of the tape, or expansion or compression of the magnetic tape, or errors in the capstan servo control used to drive that tape may result in time fluctuations or variations which are commonly referred to as time base errors. This problem is particularly acute when the recorded signal is a digital signal, such as a digitized representation of video or audio information. In view of such time base errors, various time base error correction systems have been designed for correcting or compensating such errors. Typically, the reproduced signal is written into a memory at a write-in rate that is synchronized with the actual rate at which the signal is reproduced. Subsequently, the stored signal is read out at a fixed, reference rate. Thus, time base errors are eliminated. Typical uses of time base correctors are found in video tape recording/reproducing systems, such as those used for industrial purposes.
Most time base correctors are digital devices which operate on digital information signals. When used in combination with a video recording/reproducing system, the usual analog video signal is converted into digital form, and the digitized video signal is supplied to the time base corrector wherein time base errors are corrected. Thereafter, the digitized video signal is reconverted back to analog form so as to be subsequently transmitted, displayed, or the like.
Recently, digital techniques have been applied to the recording of audio signals. For example, so-called PCM recorders have been proposed, wherein the audio signal is converted into digital form, such as a pulse code modulated (PCM) signal, and this PCM signal then is recorded. Since the PCM audio signal may be subjected to time base errors during reproduction, time base correctors have been proposed for such error correction or compensation. One example, of such a time base corrector is disclosed in U.S. Pat. No. 4,141,039.
In addition to time base errors, signals which are recorded and reproduced from a magnetic medium may be subjected to distortion, dropout, and other errors which, generally, are inherent in magnetic media. Such errors are particularly noticeable when digital signals are recorded and reproduced. This is because the obliteration of even a portion of the digital signal may have a magnified distortional effect when that digital signal subsequently is converted back into analog form. Consequently, in order to minimize such delterious effects due to these errors, various error correction encoding techniques have been proposed for use in recording digital signals. One such technique is known as the time-interleaved error correction encoding technique and is described in copending application Ser. No. 195,625, filed Oct. 9, 1980. In the time-interleaved error correction encoding technique, digital words which are temporarily spaced far apart are selected and combined in a data block. Thus, such data words are "time-interleaved" with each other. Such time-interleaving generally is carried out in stages, with a parity word being generated at each stage. Such parity words also are time-interleaved, resulting in a data block formed of data and parity words derived from substantially time-separated information. With this technique, if a particular data word or an entire data block is obliterated, the actual effect is to destroy isolated data words which can be reconstructed either by conventional error correction techniques (such as parity techniques), or may be "concealed" by replacing the destroyed word with a simulated word that is produced by interpolating those "good" words which precede and follow it.
While the time-interleaved error correction encoding technique is a powerful tool for minimizing the effect due to errors in recording/reproducing systems, it is important that, during time base correction thereof, the proper sequence of reproduced data blocks is maintained. Typically, when data blocks 1, 2, 3 and 4 are reproduced, they may be stored in storage locations 1, 2, 3 and 4, respectively, of the usual memory included in a time base corrector. Such storage is temporary and, during the subsequent read-out operation, storage locations 1, 2, 3 and 4 are read out in sequence. Thus, the very same sequence of data blocks that has been reproduced from the record medium and stored in memory is read out therefrom. It is important, therefore, that when data block 1 is reproduced from the tape, it is stored in storage location 1 rather than storage location 2. Likewise, all of the remaining data blocks should be stored in their proper storage locations.
Various possibilities exist, however, which give rise to the storage of a data block in an incorrect storage location, thereby changing the sequence in which such data blocks are read out from the memory of the time base corrector. For example, if each data block includes a preface synchronizing signal which is used to increment a write address generator, spurious signals may be erroneously interpreted as such a synchronizing signal, thereby changing the write address improperly. Alternatively, if the synchronizing signal is not detected because of, for example, dropout, the write address will not change, and the data block will be stored in an improper location.
Another difficulty associated with time base correction of time-interleaved data blocks is that associated with the detection of an error in a reproduced data block. In some time base correctors, if a reproduced data block is erroneous, it is not written into its assigned stage location in the memory. Rather, the preceding data block already stored in that location or, alternatively, the immediately preceding data block which had been reproduced from the record medium, is stored in such assigned location. Then, when the memory subsequently is read out, a redundant data block will be read, rather than reading out an erroneous data block therefrom. While this technique generally is satisfactory if the data blocks represent slowly changing information, this technique offers less than satisfactory results when the data blocks are formed of time-interleaved data words. There is a need, therefore, to prevent a data block which has already been read out from the time base corrector memory from being read out once again.